1. Field
The present disclosure relates to thin film transistors (TFTs) and/or methods of driving the same, and more particularly, to TFTs capable of improving an on/off duty ratio and/or methods of driving the same.
2. Description of the Related Art
Thin film transistors (TFTs) are widely used as switching devices or driving devices in an electronic device field. For example, a TFT may be used as a switching device in a pixel of a display.
When a typical TFT is used as a switching device in a pixel of a display, a turn-off time of the TFT is much longer than a turn-on time thereof. Thus, an on/off duty ratio of the TFT is very small. That is, a turn-off voltage is applied to a gate of the TFT for almost all time.
For example, for an active matrix display using TFTs, a TFT is turned on by applying a turn-on voltage to one gate wiring connected thereto for a time period obtained by dividing a time allocated to one frame (1/frame frequency) by the total number of gate wirings, thereby delivering a voltage or a current therethrough.
For example, an on/off duty ratio for 120 Hz and a full high definition (FHD) (1920*1080 pixels) resolution is calculated as follows. One frame (= 1/120) is 8.3 msec, and a turn-on time of a TFT is 8.3 msec/1080=7.7 μsec, and thus, the on/off duty ratio is about 0.001, i.e., about 0.1%.
As described above, a TFT is turned on only for a very short time during one frame and is turned off for the remaining time during which the TFT does not operate. This turn-on/off operation repeats with respect to each consecutive frame. Thus, an on/off duty ratio of the TFT is very small.
For a typical n-type TFT, a negative voltage is applied to a gate during a turn-off time, which occupies almost all time of one frame. Accordingly, a time period during which holes are trapped in an interface between a gate insulating layer and an active layer or inside the gate insulating layer is longer than a time period during which the holes are detrapped. As time goes by, trapped holes are accumulated. Thus, a threshold voltage of the TFT moves to a negative direction, thereby deteriorating the reliability thereof. On the contrary, for a p-type TFT, a positive voltage is applied to a gate during a turn-off time, which occupies almost all time of one frame. Accordingly, a time period during which electrons are trapped in an interface between a gate insulating layer and an active layer or inside the gate insulating layer is longer than a time period for which the electrons are detrapped. As time goes by, trapped electrons are accumulated. Thus, a threshold voltage of the TFT moves to a positive direction, thereby deteriorating the reliability thereof.
As described above, when an on/off duty ratio of a TFT is small, holes or electrons are trapped in an interface between a gate insulating layer and an active layer or inside the gate insulating layer. As time goes by, a threshold voltage of the TFT moves, thereby deteriorating the reliability thereof.